1. Field of the Invention
This invention relates to high power electrochemical capacitors comprised of high energy density electrode materials. In particular, this invention relates to electrode structures comprised of composite electrode materials and an electrolyte.
2. Prior Art
Electrochemical capacitors are devices which store electrical energy at the interface between an ionically-conducting electrolyte phase and an electronically-conducting electrode material. Electrochemical capacitors were first described in a 1957 patent by Becker, listed infra. The first practical devices were pioneered by SOHIO and described in U.S. Pat. No. 3,536,963. These devices were based on the double-layer capacitance developed at the interface between high-area carbon electrodes and sulfuric acid electrolyte solution. A complementary system originating from a different electrochemical phenomenon, i.e. the development of pseudocapacitance associated with a surface reaction, was developed by Conway in 1975, in collaboration with Continental Group, Inc. See Can. Pat. by Craig, which is also listed infra. The materials possessing pseudocapacitance discovered in Conway et al.'s work are metal oxides which include ruthenium oxide, iridium oxide, cobalt oxide, molybdenum oxide, and tungsten oxide. The most effective material discovered was ruthenium oxide which gives a reversibly accessible pseudocapacitance of many Farads per gram over a 1.4 volt range.
Composite electrode structures have previously been used in electrochemical capacitors and batteries. For example, Tatarchuk et al in U.S. Pat. Nos. 5,080,963, 5,096,663 and 5,102,745 describe an electrode comprised of highly conductive metal fibers and highly porous carbon fibers. The carbon fiber is used as the active material which contributes the capacitance for charge storage and which has a low electrical conductivity. The metal fiber is highly electrically conductive and was used to form a network for reducing the resistance of the electrode. In U.S. Pat. Nos. 5,065,286, 5,086,373 and 5,121,301, Kurabayashi et al describe a polarized electrode structure which is similar to the composite electrode structure mentioned above. The polarized electrode structure comprises an active material and a honeycomb type current collector. The honeycomb type current collector is used to increase the contacting area between the active material and the current collector and, therefore, the resistance of the electrode is reduced.
Malaspina, in U.S. Pat. No. 5,079,675, describes another type composite electrode for electrochemical capacitors. This electrode is formed by coating metal oxides onto particles of a high surface area material such as carbon black. The purpose of coating the metal oxides onto the carbon is to facilitate fabrication of the electrodes.
For many batteries' electrodes, carbon black is mixed with active materials, such as MnO.sub.2, CoO.sub.2, SO.sub.2, Li.sub.x Co.sub.1-x O.sub.2, Li.sub.x Mn.sub.1-x O.sub.2, and Li.sub.x V.sub.1-x O.sub.2. A description of such battery electrodes is found in a reference by Linden listed infra. The carbon black serves the dual purpose of increasing the electrical conductivity and the porosity of the electrode. Because carbon is more electrically conductive and is more porous than those active materials which are semiconductors, ions in the electrolyte react quickly. Therefore, both effects enhance the battery's performance at high rates.
The following is a list of relevant prior art:
______________________________________ U.S. PATENT DOCUMENTS 3,536,963 /1957 Becker 5,065,286 12/1991 Kurabayashi et al. 5,072,336 12/1991 Kurabayashi et al. 5,079,674 01/1992 Malaspina 5,080,963 01/1992 Tatarchuk et al. 5,086,373 02/1992 Kurabayashi et al. 5,096,663 03/1992 Tatarchuk 5,102,745 04/1992 Tatarchuk et al. FOREIGN PATENT DOCUMENTS 1,196,683 06/1990 Canada ______________________________________
OTHER PUBLICATIONS D. Linden, "Handbook of Batteries and Fuel Cells", McGraw-Hill 20- Book Co., New York, 1984.